Implementation of function wrappers

[mmikhasenko]

For the validation of the functions RUB-EP1/amplitude-serialization#44, follow up on discussion with @IlyaSegal

The functions in serialization format know about the name of their argument, they should be able to be called on a Dictionary argument, similar

The information about the argument should be attached to the function itself.
it won't be clean to implement it to HadronicLineshape. The functionality belongs here, or in a separate package HadronicLineshapesIO.jl.

The suggesting to implement a wrapper around AbstractFlexFunctions similar to HadronicUnpolarizedIntensity,
that keeps info about the argument. The dict2object method would rather return this new type of object.